When processing a metallic sheet having a large area and a complicated curved surface, such as an aircraft wing, a processing method called peen forming or shot peen forming has been widely used in recent years. As disclosed in, for example, Patent Literatures 1 and 2, this processing method involves retaining a metallic plate-like workpiece and blasting the plate-like workpiece with steel particles called shot having a diameter of about 0.5 mm to 4 mm at high speed so as to make the steel particles collide therewith with large momentum, thereby generating plastic strain in the plate-like workpiece and curving and shaping the plate-like workpiece into a desired shape.
When performing such peen forming, it is known that, by blasting the plate-like workpiece with shot after preliminarily twisting the plate-like workpiece within its elastically deformable range or after curving and retaining the plate-like workpiece, elastic stress applied to the plate-like workpiece accelerates deformation of the plate-like workpiece and thus significantly enhances the shapeability thereof. This technique is called stress peen forming. When performing this stress peen forming, the plate-like workpiece is elastically deformed by forcedly making the plate-like workpiece conform to a press jig and securing the plate-like workpiece with a securing tool, as disclosed in FIG. 8(b) of Patent Literature 2.
Furthermore, without the use of a press jig, a plate-like workpiece is retained in a twisted or curved shape by using a clamping-type retaining apparatus or a retaining apparatus that employs a hydraulic jack. In this case, in order to apply stress that gives the peened plate-like workpiece a curvature the same as or close to that of the completed shape thereof by using the aforementioned retaining apparatus, a template (i.e., an R-shaped template) having a predetermined curvature that is set in view of spring-back is pressed against the curved surface of the plate-like workpiece while checking the curvature thereof so as to set it to the predetermined curvature.